Alloys for high-temperature service



ALLOYS FOR HIGH-TEMPERATURE SERVICE James F. Baldwin, New York, N.Y. (Chicken Valley Road, Locust Valley, N .Y.)

No Drawing. Filed Dec. 11, 1958, Ser. No. 779,537

8 Claims. (Cl. 75-126) This invention relates to alloys suitable for service at high temperatures, and is particularly directed to the provision of hardenable alloys which in the annealed condition can be worked by conventional machining and forging operations, and which in the hardened condition are capable of retaining their hardness at temperatures up to 1100 F. or even higher. The new alloys are moderately high carbon ferrous alloys containing chromium, tungsten and cobalt as essential ingredients. In addition to possessing high hot-hardness and resistance to wear at elevated temperatures, they display a high degree of resistance to scaling and other forms of deterioration by oxidation at high temperatures and excellent resistance to corrosion. I

It has long been known that moderately high carbon steels containing chromium (eg 17% by weight chromium and 0.6% to 1% by weight carbon) can be hardened by quenching from an elevated temperature, and show fairly good resistance to corrosion and to scaling due to oxidation at high temperatures. Such alloys, however, do not retain the hardness developed by heat treatment when they are reheated to moderately high temperatures for extended periods of time. It has also long been known that tungsten-bearing tool steels, which typically contain about 18% tungsten, are hardenable by quenching from a high temperature, and in the hardened condition show a high degree of resistance to softening when reheated to quite elevated temperatures. These alloys, however, possess only poor resistance to corrosion and to scaling due to oxidation when thus reheated.

Attempts to improve the hot-hardness of the chromium steels by adding tungsten, or to improve the corrosion and oxidation resistance of the tungsten tool steels by adding chromium, have not provide successful. The presence of sufiicient tungsten and chromium to insure ,both high hot-hardness and high corrosion and oxidation resistance produces an alloy in which the ferritic structure' is so effectively stabilized that the steel loses its ability to be hardened by heat treatment.

I have found that the addition of cobalt togethe with tungsten, in proper proportions, to a moderately high carbon chromium-bearing steel results in an alloy which can be hardened by heat treatment, and which possesses both a high degree of hot-hardness and resistance to wear at elevated temperatures, and high corrosion and scaling resistance at elevated temperatures. In order to retain the property of being hardenable by heat treatment (i.e. by rapid cooling or quenching from a high temperature), the amount of cobalt should not exceed the amount of tungsten by more than a few percent if at all, and advantageously the amount of tungsten should not exceed the amount of cobalt by more than a few percent.

The alloys of this invention accordingly are composed essentially of from 0.9% to 1.75% by weight of carbon, from 13% to 20% by weight of chromium, from 3% to 12% byweight of tungsten, from 3% to 12% by weight of cobalt, the amount of tungsten usually being at least atcnt C about equal to the amount of cobalt, and the balance iron. Preferably the amount of tungsten is in the range from equal to up to 5% greater than the amount of cobalt in the alloy. Generally, the carbon is present in the narrower range from 1% to 1.3% by weight; and most advantageously the alloy contains from 9% to 12% by weight tungsten and from 7% to 10% cobalt, the amount of tungsten being substantially 2% by weight greater than the amount of cobalt. Particularly satisfactory alloys contain about 11% by Weight of tungsten and about 9% by weight of cobalt.

These alloys are characterized by being hardenable by rapid coolingor quenching from a temperature in the range from 1800 F. to 2050 F. In the hardened condition they have a hardness of at least 50 and up to 65 on the Rockwell C scale, and they are capable of retaining such hardness and are resistant to scaling or other form of deterioration by oxidation when heated for prolonged periods at temperatures up to 1100 F.

Alloys according to the invention not only possess high hot-hardness and are oxidation-resistant at elevated temperatures, but also display a high degree of resistance to wear and abrasion even when highly heated. The high temperature properties of the new alloys are superior in all respects to the hardenable carbon-bearing chromium steels known heretofore. The new alloys are more corrosion-resistant, more resistant to scaling and oxidation at high temperatures, much more resistant to softening when heated, and notably more resistant to abrasion and wear than the heretofore known hardenable chrome steels. Moreover, the new alloys are much more corrosion and oxidation resistant than the tungsten-bearing tool steels known heretofore, and therefore are much more suitable for elevated temperature use.

Attainment of these advantageous properties in an alloy which is hardenable by heat treatment is dependent on maintaining a proper balance between the tungsten and cobalt. Ifthe cobalt content exceeds the tungsten content by as much as 4% or more, the hardenability of the alloy is very considerably impaired. If the cobalt and tungsten contents are substantially equal, the alloy is found to possess fairly good hardening characteristics. Optimum hardening is achieved when the alloy contains about 2% by weight more of tungsten than of cobalt. When the tungsten content exceeds the cobalt content by substantially more than 5%, hardenability of the alloy diminishes again from its optimum level. If the combined amount of cobalt plus tungsten exceeds 24%, the alloy substantially loses its ability to be hardened by heat treatment. I I

As an example, an advantageous alloy according to the invention contains nominally 1.0% by weight carbon, 17% by weight chromium, 11% by weight tungsten, 9% by weight cobalt, and the balance iron. The alloy is not particularly sensitive to variations within the stated limits of 0.9% to 1. 75% carbon and 13% to 20% chromium. For best results, however, the tungsten and cobalt contents should be as close as possible to the upper limits of the range of 3% to 12% for each of these components, provided, of course, that the proper balance is maintained between them. For example, in alloys containing 1% by weight carbon and 17% by weight chromium, best results are attained when the amount of tungsten present is 11% and the amount of cobalt present is 9%. When the amounts of these elements are reduced to 10% by weight tungsten and 8% by weight cobalt, the afore mentioned desirable properties of the alloy are still very good, but less than optimum. Continued diminution of the tungsten and cobalt concentrations within the permissible limits, even when they are maintained in proper balance, results in "some loss in desirable properties, but

not to such an extent as to cause the alloy to lose its usefulness for high temperature service where a considerable degree of hot-hardness is required.

Similarly it is possible to depart from the optimum balance between tungsten and cobalt. For example, in an alloy containing 1% carbon, 17% chromium, and 9% tungsten, excellent hardening response to heat treatment and retention of hardness at elevated temperatures is attained when the alloy contains about 7% cobalt. If the cobalt content is decreased to or increased to 9%, the ability of the alloy to be hardened by heat treatment and to retain the hardness thus developed at high temperatures is diminished, but not so much as to destroy the usefulness of the alloy.

Besides the components discussed above, the alloy may optionally contain other ingredients which do not essentially alter its characteristics and which may even impart some improved properties to it. In addition to impurities usually present in the iron and other principal components, the alloy may optionally contain up to 2.5% by weight of copper, up to 1% by weight each of any, some or all the metals nickel, vanadium, titanium, silicon and manganese, up to 0.1% nitrogen, and up to 0.01% by weight of boron. Of these optional ingredients, the presence of copper improves workability of the alloy, a small amount of titanium appears to improve its corrosion resistance, nitrogen is helpful because it stabilizes the austenitic structure and thereby helps to eliminate the weak ferrite structure in the final hardened condition, and boron even in the very small amount indicated evidently improves its workability.

Alloys according to the invention may be employed in either the cast or wrought form. In the ascast and slowly cooled condition, the new alloys typically have a hardness of about 28 to 35 on the Rockwell C scale. Cast ingots may be worked hot (at say 2150 F.) by swaging, forging, rolling and other conventional procedures to wrought shapes. Both cast and wrought forms of the new alloy, in the as-cast or annealed condition having a Rockwell C hardness of 28 to 45, may be machined and otherwise fabricated.

The new alloys are hardened by heating to a temperature preferably in the range 1800 F. to 2000 F. and then rapidly cooling from such temperature in air or quenching in oil, water, or a Dry-Ice chilled liquid. The hardened alloy has a hardness ranging from 50 to 65 on the Rockwell C scale. The hardened alloy may be tempered to relieve strains by reheating to a temperature in the range from 900 F. to 1300 F. without serious loss of hardness developed by the preceding heat treatment. Even after prolonged tempering at 1200" F., the hardness of the alloy remains near or above 50 Rockwell C. Secondary tempering at a temperature which may be lower than, the same as, or higher than the primary tempering temperature is sometimes desirable. To anneal the alloy, either to relieve work hardening or to render heat treated metal workable, it is heated to a temperature above 1800 F. and then is slowly cooled to room temperature.

The susceptibility of the new alloys to hardening by heat treatment similar to that used for hardening other moderately high carbon steels, plus its ability to retain at 1100 F. the hardness developed by heat treatment, renders them especially well suited for making parts which must possess a high degree of hot-hardness in use. The ability of the alloys to be Worked and machined in either the wrought and annealed or as-cast condition, and then to be hardened by heat treatment, is of course a particularly desirable property which facilitates fabrication of intricate parts, or parts which must be made to close dimensional tolerances. The high resistance of the new alloys to corrosion and to scaling or other injury due to oxidation, and their resistance to wear and abrasion at high temperature, are important virtues which enable accurately shaped moving parts to be operated at elevated temperatures without damage.

While there are numerous uses to which the new alloys may be put, an example of one such use for which they have proved eminently suited is as bearings for the hightemperature end of aircraft gas turbines or jet engines. Such bearings must be accurately machined or otherwise formed, they must possess and retain a high degree of hardness at the high temperatures (approaching 1100 F.) to which they are subjected in use in order to Withstand deformation under the forces to which they are subjected, and they must of course resist corrosion, scaling or other injury due to oxidation, and abrasion or erosion at such high temperatures. Such bearings made of the alloys according to this invention meet all these requirements very satisfactorily.

-I claim:

1. An alloy composed essentially of from 0.9% to 1.75% by weight carbon, from 13% to 20% by weight chromium, from 3% to 12% by weight tungsten, from 3% to 12% by weight cobalt, the amount of tungsten being at least about equal to the amount of cobalt, and the balance iron, said alloy being hardenable by rapid cooling from 1800 F.

2. An alloy composed essentially of from 0.9% to 1.75% by weight carbon, from 13% to 20% by weight chromium, from 3% to 12% by weight tungsten, from 3% to 12% by weight cobalt, and the balance iron, the amount of tungsten being from at least equal to up to 5% greater than the amount of cobalt, and said alloy being hardenable by rapid cooling from 1800 F.

3. An alloy composed essentially of from 1% to 1.3% by weight carbon, from 13% to 20% by weight chromium, from 9% to 12% by weight tungsten, from 7% to 10% by weight cobalt, the amount of tungsten being substantially 2% by weight greater than the amount of cobait, and the balance iron, said alloy being hardenable by rapid cooling from 1800 F.

4. An alloy composed essentially of from 1% to 1.3% by weight carbon, from 13% to 20% by weight chromium, about 11% by weight tungsten, about 9% by weight cobalt, and the balance iron, said alloy being hardenable by rapid cooling from 1800 F.

5. An alloy composed essentially of from 0.9% to 1.75% by weight carbon, from 13% to 20% by weight chromium, from 3% to 12% by weight tungsten, from 3% to 12% by weight cobalt, the amount of tungsten being at least about equal to the amount of cobalt, up to 2.5% by weight of copper, up to 1% by weight each of nickel, vanadium, titanium, silicon, and manganese, up to 0.1% nitrogen, up to 0.01% boron, and the balance iron, said alloy being hardenable by rapid cooling from 1800 F.

6. An alloy composed essentially of from 1% to 1.3%

by weight carbon, from 13% to 20% by weight chromium, from 9% to 12% by weight tungsten, from 7% to 10% by weight cobalt, the amount of tungsten being substantially 2% by weight greater than the amount of cobalt, up to 2.5 by weight of copper, up to 1% by weight each of nickel, silicon and manganese, up to 0.1% nitrogen, up to 0.01% by weight boron, and the balance iron, said alloy being hardenable by rapid cooling from 1800 F.

7. A hardened alloy having a hardness of at least 50 on the Rockwell C scale composed essentially of from 0.9% to 1.75% by weight carbon, from 13% to 20% by weight chromium, from 3% to 12% by weight tungsten,- and from 3% to 12% by weight cobalt, the amount of tungsten being at least about equal to the amount of cobalt, and the balance iron, said alloy being capable of retaining said hardness and being resistant to deterioration by oxidation at temperatures up to 1100 F.

8. A hardened alloy having a hardness of at least 50 on the Rockwell C scale composed essentially of from 1% ,to 1.3% by weight carbon, from 13% to 20% by vanadium, titanium,

weight chromium, from 9% to 12% by weight tungsten, from 7% to 10% by weight cobalt, the amount of tungsten being substantially 2% by weight greater than the amount of cobalt, and the balance iron, said alloy being capable of retaining said hardness and being resistant to deterioration by oxidation at temperatures up to 1100 F.

References Cited in the file of this patent UNITED STATES PATENTS Fahrenwald Nov. 2, 1920 Nelson Oct. 14, 1930 Payson Dec. 20, 1938 Kirkby et a1 Mar. 31, 1959 

1. AN ALLOY COMPOSED ESSENTIALLY OF FROM 0.9% TO 1.75% BY WEIGHT CARBON, FROM 13% TO 20% BY WEIGHT CHROMIUM, FROM 3% TO 12% BY WEIGHT TUNGSTEN, FROM 3% TO 12% BY WEIGHT COBALT, THE AMOUNT OF TUNGSTEN BEING AT LEAST ABOUT EQUAL TO THE AMOUNT OF COBALT, AND THE BALANCE IRON, SAID ALLOY BEING HARDENABLE BY RAPID COOLING FROM 1800*F. 